For guiding laser beams, it is customary to use either light conductors in the form of flexible glass fibers or transmission systems containing optical deflectors such as mirrors. Particularly in medical applications such as laser surgery, for example, articulated transmission systems of the last-mentioned type are frequently used because so far there are no fiber cables which are sufficiently resistant to rupture while at the same time providing for a sufficiently low-loss transmission in the wavelength range of the carbon-dioxide lasers predominantly used in these applications.
The articulated arms used for guiding the laser beam include a series of deflection mirrors secured to rotatable joints such that each mirror is rotatable about the axis of the incident laser beam together with the follow-on part of the pivot arm. In addition, telescope guides are often inserted between the joints thereby enabling the length of the arm to be changed. Articulated arms having as many as seven rotatable joints are known.
Apart from being dependent on a sufficiently precise adjustment of the mirrors, the proper function of such an articulated arm depends on the quality of the pivot bearings and telescopic guides as well as on the rigidity of their connections. The problems associated therewith become greater as the dimensions of the work area covered by the articulated arm increase. Adding more rigidity to the arm requires stronger material and, accordingly, larger masses to be moved. This puts an increased load on the pivot bearings and impairs handling, particularly in dynamic operation.
If, however, guidance errors occur, the beam will wander from its predetermined beam path. First of all, this will cause the laser focus to shift which is not particularly disturbing within certain limits, especially if the focus position is visually evaluated by means of a reflected pilot beam. Where long articulated arms are used, the situation can occur that the laser beam within the articulated optical system wanders out of the free openings of the deflection mirrors or the focusing optics and impinges on their mountings. In this event, high-performance lasers will destroy the optical system.
In laser systems for reading out the contents of information storage plates, it is known to provide control arrangements which make the focus of the laser beam follow a predetermined track radially. These control arrangements, however, evaluate the deviation of the focus from the visibly marked data track. In addition, there exists a fixed spatial relationship between the direction of movement of the positioning member and that of the laser focus. However, if the laser beam is guided via several articulated joints, the angle of rotation of each individual joint has to be taken into consideration when determining the relationship between the position coordinates of the focus and the positioning coordinates of the member performing the follow-up function. This is not easily possible since the complexity of the arrangement would increase considerably if each articulated joint were provided with an angle sensor of its own, particularly in articulated arms including several pivot axes.